Waco Gem and Mineral Club, Waco, Tx

17.5 Natural Fiber Optics or TV Stone


17.5: Natural Fiber Optics, or 'TV Stone'

Near the town of Boron, California, is an immense open-pit mine where truckload after huge truckload of borate minerals have been dug for generations for commercial applications; for instance, to produce borax for laundry detergent. These minerals were concentrated in a closed basin as ancient lakes dried up in the desert. One of those borate minerals is called ulexite, for the German chemist who first discovered it in the 1800s, Georg L. Ulex.

Ulexite is composed of long, thin crystals that might grow as fluffy, cotton-like puffballs or, more commonly, as compact, blocky masses of fibrous veins, with crystals tightly aligned side-by-side. The fibrous crystal bundles give blocky masses of white ulexite a soft, satin luster.

To demonstrate a truly neat special effect, use a rock saw to cut a chunk of ulexite perpendicular to those crystal bundles at top and bottom. Then polish both top and bottom. (You can often find small specimens of ulexite in rock shops and museum gift shops already cut and polished.) Now have your kids place that chunk onto the words in a book or atop a colorful drawing on the comics page of the Sunday newspaper. The words or picture will seem to be sucked up and will appear at the top surface of the ulexite, just like an image on a television screen!

The individual crystals making up the block of ulexite act like fiber-optic cables. Each transmits light from the bottom surface of the stone to the top surface, thus producing the unique optical property that earned ulexite its nickname of "TV stone".